The loose-knit group of online global hackers known as "Anonymous" has trained its sights on Yemen following cyber attacks on government websites in Tunisia and Egypt.
The website of Yemen President Ali Abdullah Saleh, presidentsaleh.gov.ye, was inaccessible on Thursday following calls by Anonymous members for attacks on the site.
Valleywag, a Silicon Valley blog owned by the Gawker network, said the cyber attacks on official Yemen government websites had been dubbed "Operation Yemen" in Anonymous chat rooms.
Tens of thousands of Yemenis staged a "day of rage" in Sanaa on Thursday against President Ali Abdullah Saleh as clashes and protests against Egyptian President Hosni Mubarak were continuing in Cairo.
The attacks on Yemeni websites came a day after Anonymous "hacktivists" resumed attacks on the websites of Egypt's ruling National Democratic Party, the Ministry of Interior, Ministry of Information and others.
The hackers first began attacking Egyptian government websites on January 26, according to computer security experts, as part of a campaign dubbed "Operation Egypt" before the authorities there shut down the Internet.
Those sporadic Distributed Denial of Service (DDoS) attacks on official Egyptian websites resumed on Wednesday after the country went back online but most Egyptian sites were accessible on Thursday.
"Welcome back to the Internet #Egypt. Well except www.moiegypt.gov.eg -- you stay down," members of Anonymous said on the Twitter feed @AnonymousIRC.
Anonymous last month managed to shut down the Tunisian government's official website, the national stock exchange site, and other sites during a popular uprising that led to the ousting of the country's dictator.
In a typical DDoS attack, a large number of computers are commanded to simultaneously visit a website, overwhelming its servers, slowing service or knocking it offline completely.
In December, members of Anonymous attacked the websites of Amazon, PayPal, Mastercard, Visa and others for withdrawing their services to WikiLeaks, the anti-secrecy website.
British police arrested five people last month and the US Federal Bureau of Investigation launched raids across the United States in connection with the cyber attacks by Anonymous members.
The defense of WikiLeaks was an extension of "Operation Payback," a movement which began on the Internet message board 4Chan in September.
Operation Payback involved cyber attacks on the websites of the Motion Picture Association of America, Recording Industry Association of America and others over their vigorous copyright protection efforts.
Explore further: Report: NSA spying on virtual worlds, online games (Update)